Hydrogen is finding a wide range of applications in present-day petroleum refining processes. The processes involving nonsaturated hydrocarbons hydrogenation in the manufacture of high-grade gasoline, aromatic hydrocarbons, olefine hydrogenation from acetylenic hydrocarbons and petroleum fractions hydrocracking are reputed to be a major consumer of hydrogen. Furthermore, high-purity hydrogen is being extensively utilized now as fuel for rocket engines.
While conducting such petroleum refining processes, and especially those concerned with the pyrolysis of diverse hydrocarbon materials, along with such components as ethylene, propylene, butane-butylenes, aromatic hydrocarbons, a fairly substantial amount of hydrogen is also being formed. The amount of hydrogen runs up to 1.5% by weight of the bulk of the material processed and therefore its recovery is assuming an ever increasing scale. The output of hydrogen recovery facilities operated in today's ethylene plants reaches 4,000+ 40,000 tons of hydrogen per year.
In the prior state of the art there are known a multiplicity of various methods of and apparatus for producing hydrogen from gaseous mixtures of hydrocarbons.
In particular, there is known a method of producing hydrogen from a gaseous mixture including primarily hydrogen, methane and olefins (cf. U.S. Pat. No. 3,443,388), according to which the gaseous mixture is cooled in a plurality of stages of successively lower temperatures to effect condensation of the olefins and a major portion of the methane from said mixture. Hydrogen is withdrawn from the last stage of cooling, and the condensate from the last two stages of cooling is introduced into a demethanizer. An amount of refrigeration required for cooling the hydrogen-rich gas is supplied by evaporization of the methane-rich condensate when throttling it and evaporizing countercurrently in indirect heat exchange contact against the gas being cooled.
The aforesaid method is incorporated in an apparatus comprising coolers, separators and a demethanizer. The demethanizer is communicated with liquid-containing sections of the separators provided for the separation of the gas-liquid streams rich in olefins. A gas-containing section of the separator provided for the separation of a methane-hydrogen fraction from the olefins is in communication via a heat exchanger wherein the liquid methane fraction throttled to a low pressure is used as a cooling agent, with the separator wherein the separation of the liquid methane and the gaseous hydrogen fractions is accomplished. The resulting hydrogen of 80-85 mole percent purities is withdrawn from the gas-containing section of this separator, and the liquid methane is withdrawn from its liquid-containing section and fed into the demethanizer via a pipeline.
The most serious disadvantage of the abovedescribed method and apparatus for the production of hydrogen consists in that the resulting hydrogen has a purity degree of not more than 80-85 mole percent which is attributable to the insufficiently low temperature level obtained at the last stage of the cooling of the gaseous mixture under separation. The throttling of the liquid methane as employed in this method affords temperatures of about -137.degree. C., at which level the production of hydrogen of higher purity degrees is impracticable.
Moreover, this prior art method and apparatus fail to take full advantage of the energetic potentialities of the compressed gaseous mixture under separation in order to gain additional amounts of refrigeration.
Also known is a method of producing hydrogen from a gaseous mixture including essentially hydrogen, methane and olefins (see U.S. journal "Petroleum Engineer", 1972, No. 3, pp. 117-122), which partly eliminates the disadvantages intrinsic to the method described previously. According to this prior art method the gaseous mixture is cooled in a plurality of stages of sequentially lower temperatures, with the most of the olefins being initially removed from the gaseous mixture by condensation and directed for demethanization, the remaining portion of the gaseous mixture comprised of hydrogen and methane being subjected to further cooling in stages for the purpose of effecting maximum methane condensation, and a balance of refrigeration needed for said cooling of the gaseous mixture comprised of hydrogen and methane being provided by expanding nitrogen in an expansion turbine. Nitrogen utilized as a cooling agent is compressed in a nitrogen compressor prior to its introduction into said expansion turbine.
This prior art method is implemented in an apparatus comprising coolers, separators, a demethanizer, a nitrogen compressor and an expansion turbine. The liquid-containing sections of the separators provided for the separation of the olefins-rich mixtures are communicated with the demethanizer via pipelines, and the gas-containing sections of said separators are adapted to be in communication with the heat exchangers wherein the nitrogen being expanded in the expansion turbine is employed as a cooling agent. The separation of the liquid methane fraction and the gaseous hydrogen fraction formed at the last cooling stage is effected means of the separator from the gas-containing section of which the resulting hydrogen of 95-97 mole percent purities is withdrawn via a pipeline, while the liquid methane is withdrawn from the liquid-containing section thereof and then directed via a pipeline for evaporization countercurrently against the gaseous mixture being separated.
Although the abovestated method of and apparatus for producing hydrogen afford the production of hydrogen of higher purities than those obtainable with the method and apparatus considered hereinafore, however, this hydrogen purity improvement is achieved at the expense of making the method and apparatus design more complex, apart from appreciably increasing power consumption. This is incidental to the use of nitrogen as a source of refrigeration which is to be compressed in a nitrogen compressor and then expanded in an expansion turbine.
In addition, this prior art method and apparatus do not employ to the full the energetic possibilities present in the gaseous mixture under separation in terms of securing supplementary refrigeration.
In the present-day state of the art of petroleum refining industry there is generally known a method concerned with the production of hydrogen from a gaseous mixture including hydrogen, methane and olefins, which method partially surmounts the disadvantages of the abovedescribed methods. According to this method, the gaseous mixture is cooled by stages in heat exchangers by the products of its separation to such temperature levels as to provide removal of the olefins and a portion of the methane from the gaseous mixture. The condensate formed at each of these cooling stages is separated from the remaining gaseous mixture and passed for demethanization. The remainder of the gaseous mixture comprised of hydrogen and methane obtained after removal of the olefins is passed for further cooling by stages for the purpose of condensing hydrogen-admixed methane from the gaseous mixture. The resulting liquid hydrogen-admixed methane is separated, subjected to throttling and then delivered for evaporization in the heat exchangers of corresponding temperatures in a countercurrent manner in relation to the gaseous mixture being separated. Hydrogen is recovered at the last cooling stage. In order to provide low temperature levels at this stage of cooling the liquid hydrogen-admixed methane is caused to be evaporized at a pressure of from 0.04 mn/m.sup.2 to 0.65 mn/m.sup.2 through the use of a methane compressor including a vacuum stage.
The abovementioned method is incorporated in an apparatus which comprises tubular heat exchangers, separators, a demethanizer and a methane vacuum stage-containing compressor. A number of the tubular heat exchangers and separators combines into a unit providing removal of the olefins from the gaseous mixture under separation, while the rest of the heat exchangers and separators combines into a unit providing the separation of the remainder of the gaseous mixture into methane-admixed hydrogen and hydrogen-admixed methane. The tubes of the heat exchangers are designed for the conveyance of the products of the separation of the remainder of the gaseous mixture into methane-admixed hydrogen and hydrogen-admixed methane. The intertubular spaces of all the heat exchangers are designed for the conveyance of the gaseous mixture under separation and are interconnected between themselves through the gas-containing sections of the separators. The liquid-containing sections of the separators included in the unit providing the separation of olefins are in communication with the demethanizer. The liquid-containing sections of the separators included in the unit providing the separation of the remainder of the gaseous mixture into methane-admixed hydrogen and hydrogen-admixed methane are adapted to be in communication with the vacuum stage of the methane compressor via one of the tubes of the tubular heat exchangers designed for evaporization of the liquid hydrogen-admixed methane at a pressure of from 0.04 mn/m.sup.2 to 0.65 mn/m.sup.2. The resulting 97 mole percent purity hydrogen is derived via a pipeline from the gas-containing section of the last separator included in the unit providing the separation of the remainder of the gaseous mixture into methane-admixed hydrogen and hydrogen-admixed methane.
With this pripr art method and apparatus affords the production of the 97 mol percent purity hydrogen with 15-20% reduction of power consumption as compared with the method employing a nitrogen compressor and an expansion turbine as described hereinabove, the level of power consumption remains still fairly high.
Furthermore, this method and apparatus implimenting this method fail to make full use of the energetic potentialities of the gaseous mixture being separated under pressure.
Besides, the apparatus comprising a compressor with a vacuum stage is exposed to the risk of break-down whenever the ambient air is leaking in the vacuum stage of the compressor.
It should be also noted that aforementioned methods and apparatus allow to obtain 85-97 mole percent purity hydrogen which do not sufficiently meet the requirements of many manufacturing processes utilizing hydrogen.
In addition these prior art methods and apparatus have a relatively sophisticated process flow diagram.